Will ISIS Fighters Driven From Mosul Launch Attacks in Europe?

The military campaign to push ISIS out of its stronghold in Mosul may come at a high price: many hundreds of battle-tested and heavily armed fighters who could flee to Europe to launch terrorist attacks against U.S. and allied interests, officials tell NBC News.

U.S. officials have long feared that this is a natural and expected result of the long-planned military operation against thousands of ISIS fighters operating out of Iraq's second-largest city, some of whom have already used chemical weapons.